Conventionally, as a method of preserving subjects to be frozen, such as cooking ingredients or food products including fish and vegetables, living bodies or biological samples, while freshness thereof is maintained for a long time, cryopreservation is being adopted; however, it is very much a situation where reduction of quality and freshness due to a change in color tone of subjects to be frozen, deterioration of taste or drip loss (separation of moisture from frozen subjects at the time of thawing) cannot be completely prevented.
The subjects to be frozen contain a large quantity of moisture including bonding water constrained to particles, such as protein comprising the subjects to be frozen, and free water that can freely travel within the subjects to be frozen. At the time of freezing, the free water freezes and ice crystals are produced and grow to increase their volumes, and the structure of cells contained in the frozen subjects is destroyed. Consequently, at the time of thawing the frozen subjects, drip loss occurs due to the destroyed structure, and it becomes difficult to restore the frozen subjects to a fresh condition as before freezing.
In general, volume expansion of ice crystals occurs when a time of passing for a temperature area of ice crystal formulation is long. Taking this point into consideration, various refrigeration units and freezing methods so as not to loose freshness, as before freezing, after thawing have been proposed. For example, a method where a subject to be frozen are rapidly cooled down so as to promptly pass through an ice crystal formation temperature area by immersing the subject to be frozen into a liquid refrigerant or by spraying a liquid refrigerant to the subject to be frozen, and volume expansion of the ice crystal is prevented can be considered.
However, in the method where a subject to be frozen are immersed into a liquid refrigerant or a liquid refrigerant is sprayed over a subject to be frozen, even though it is possible to rapidly cool down a surface layer of the subject to be frozen, a frozen layer tends to be formed only on the surface layer. Then, a rate of cooling of the inside of the subject to be frozen is controlled according to heat transfer from the surface; however, it is delayed because the heat transfer is inhibited due to a presence of the ice crystallized frozen layer on the surface layer, and there is a problem where the volume expansion of the ice crystals cannot be effectively prevented inside the frozen subject.
With regard to this problem, for example, in Patent Literature 1, a super-rapid refrigeration unit having a freezer that can cool down peripheral temperature of contained frozen subjects to from −30 degrees C. to −100 degrees C, and a magnetic field generation means including a static magnetic field generation means that makes a magnetic field, which unidirectionally fluctuates, act on a subject to be frozen and a dynamic magnetic field generating means is proposed.
Further, for example, in Patent Literature 2, in order to solve the problem in Patent Literature 1, in other words, to solve the problem where a variable magnetic field lacks in uniformity and an effect of the variable magnetic field is not uniformly demonstrated to a subject to be frozen and deterioration in partial quality is confirmed, application of a uniform variable magnetic field to the subject to be frozen is proposed.    Patent Literature 1: pamphlet of International Publication No. 01/24647    Patent Literature 2: Japanese Patent Application Laid-Open No. 2003-139460